1. Field of the Invention
The present invention is generally related to communication systems. More particularly, the present invention is related to cable modem systems and methods for transferring data between a cable modem system and a cable modem termination system.
2. Background
In conventional cable modem systems, a hybrid fiber-coaxial (HFC) network provides a point-to-multipoint topology for supporting data communication between a cable modem termination system (CMTS) at the cable headend and multiple cable modems (CM) at the customer premises. In such systems, information is broadcast downstream from the CMTS to the cable modems as a continuous transmitted signal in accordance with a time division multiplexing (TDM) technique. In contrast, information is transmitted upstream from each of the cable modems to the CMTS as short burst signals in accordance with a time domain multiple access (TDMA) technique. The upstream transmission of data from the cable modems is managed by the CMTS, which allots to each cable modem specific slots of time within which to transfer data.
Conventional cable modem systems are asymmetrical in that there is considerably less bandwidth available for upstream transmissions than there is for downstream transmissions. This lack of upstream bandwidth is further exacerbated by the fact that the upstream channels must be shared by multiple cable modems. As a result, the conservation of upstream bandwidth is imperative in order to maintain overall system performance. This is particularly true where cable modem users are engaging in activities that require both substantial upstream and downstream bandwidth, such as IP telephony, video teleconferencing and Internet gaming.
Conventional cable modem systems utilize DOCSIS-compliant equipment and protocols to carry out the transfer of Protocol Data Units (PDU) data packets between multiple cable modems and a CMTS. PDU data packets are comprised of a header portion and a payload portion. The payload portion is the information intended to be transmitted from one point to another, for example, voice or data. The header portion contains protocol information identifying the source and destination of the payload. The header portion of the PDU data packet further provides instructions on how to process the payload portion contained therein. The term DOCSIS (Data Over Cable System Interface Specification) generally refers to a group of specifications published by CableLabs that define industry standards for cable headend and cable modem equipment. In part, DOCSIS sets forth requirements and objectives for various aspects of cable modem systems including operations support systems, management, data interfaces, as well as network layer, data link layer, and physical layer transport for data over cable systems. The most current version of the DOCSIS specification is DOCSIS 1.1.
It has been observed, however, that the use of proprietary data transfer protocols that extend beyond those provided by the DOCSIS specification may be advantageous in conserving network bandwidth in a cable modem system. This is particularly true with respect to Payload Header Suppression (PHS). PHS, as defined by DOCSIS 1.1, allows for the suppression of unnecessary Ethernet/IP header information in the header portion of a DOCSIS packet by the cable modem and subsequent reconstruction of the header portion by the CMTS. The goal of PHS is to reduce the number of bits transferred per packet, thereby improving network bandwidth utilization. However, DOCSIS PHS only permits header suppression based on the presence of redundant header bytes in sequentially-transmitted packets. The above referenced patent applications disclose ways to utilize more efficient payload header suppression techniques in transferring data over a cable modem network. However, it is has been further observed that the DOCSIS protocol does not support data compression in the payload portion of the PDU data packets. Many packets normally transmitted in the upstream direction in a DOCSIS network contain identical ASCII character strings in the payload. Examples of these strings are “http://www.”, “POP”, “SMTP”, “GET”, and “PUT”. The network could be used more efficiently if the payload of a given DOCSIS packet could be transmitted with fewer bytes.
Heretofore, the use of proprietary data transfer protocols that extend beyond those provided by the DOCSIS specification have been avoided. This is due, in part, to the fact that the DOCSIS specification does not provide a mechanism for using alternative protocols in a cable modem system. For example, the DOCSIS specification does not provide a mechanism for the use of data packet formats other than those it provides. Moreover, because conventional CMTS and cable modem devices have been designed in accordance with the DOCSIS specification, the use of extended protocols has been avoided to ensure interoperability between individual cable modem system components. For example, conventional DOCSIS-compliant CMTS equipment is incapable of differentiating between standard DOCSIS traffic and traffic transmitted in accordance with an extended protocol.
Accordingly, what is desired is a system and method for transferring data in a cable modem network that supports the use of protocols that extend beyond the DOCSIS specification. More particularly, the desired system and method should support the use of PDU payload data compression. However, the desired system and method should be interoperable with DOCSIS in the sense that components of a cable modem system that support PDU payload compression can exist on the same network with components that do not. Furthermore, the desired system and method should require very little modification to existing cable modem system components, such as existing cable modem and CMTS equipment.